Black liquor is the spent cooking liquor from the kraft process when digesting wood into pulp removing lignin, hemicelluloses and other extractives from the wood in order to free the cellulose fibers.
The black liquor contains more than half of the energy content of the wood fed into the digester. It is normally concentrated to 65-80% by evaporators and burned in a recovery boiler to produce energy and recover the cooking chemicals. U.S. Pat. No. 4,929,307 describes a process for treating black liquor with heat in order to improve the evaporability of black liquor.
Lignin which is present in the black liquor is a complex chemical compound derived from wood, and an integral part of the secondary ceil walls of plants. Lignin plays a crucial part in conducting water in plant stems.
Lignin is a by-product of pulping processes of cellulosic raw material. There are various pulping processes such as kraft, organosolv or soda pulping. There are also various natural cellulosic raw materials which are used in such pulping processes. The cellulosic raw material and pulping processes are typically selected and optimized for the cellulose products produced, such as paper or board. The characteristics of the lignin are a result of the used raw material and the variation in that material as well as on the variation in the pulping process. Consequently, there is a big variation in the characteristics of black liquor and the lignin depending on which cellulosic raw material used, i.e. hardwood or softwood, as well on the pulping process.
There are different end uses for precipitated lignin which has been removed from, black liquor. It may be burned as a fuel or be used as a component or additive in chemical processes and products.
Today, there exist several different processes for the separation and precipitation of lignin from black liquor. One example is described in WO2006031175 A. This process for precipitation of lignin does not give the possibility to control or modify the viscosity of the precipitated lignin. However, control of viscosity, and a decrease of viscosity in particular, may be important in subsequent process steps for further processing of precipitated lignin.
There is thus a need for an improved, process for the production of lignin with improved properties, such as reduced viscosity.